We have owned both models and started with the 19. We switched for a few reasons. 1. In the 19 the person sleeping in the back of the bed has to climb over the other person to get to the bathroom in the middle of the night. I never fell, but there were times I thought I was trying out for Cirque du Soleil. With the 21C you can slide down to the bottom of the bed and get up that way - much easier.
2. The 21C is several inches wider. Not a huge deal we thought until we got the 21C, but now we (my wife, myself, and our Golden Retriever) can all scoot around each other much more easily. In the 19, we had to learn a sort of traffic pattern and take turns getting from the table to the refrigerator.
3. We really like the U-shaped dinette option in the 21C. Not sure if that is available for the 19 or not.
The advantages of the 19 are that it is smaller, and between the two the 19 is easier get around tight spots. Of course if you are worried about getting your trailer into tight spaces, I'd consider the 17. I got about 3/4 to 1 mile per gallon better gas mileage towing the 19 vs. the 21C.
Extra cabinet space and counter space are also big bonuses, and the bath area is a bit bigger on the 21C as well.
For us, we are very glad we switched for the above mentioned reasons, but those may not be a factor for you.